Power supply units for driving a mainly capacitive load, e.g., an LCD display, often perform periodic charging and discharging of the capacitive load. In battery-driven applications, the power supply unit may comprise a boost converter for supplying an H-bridge.
FIG. 1 is an exemplary power supply unit 2 comprising a boost converter 4 and an H-bridge 6. The input terminal VIN is supplied with a power supply voltage VBAT, e.g., from a battery. The typical boost converter 4 configuration comprises an inductance LB that is coupled to the power input node VIN and further to a switch SB that is controlled by a suitable control unit CTL. Further, the boost converter 4 comprises a diode DB that is coupled in forward direction between a source of the switch SB and the inductance LB, respectively, and an output capacitor CO. The anode of the diode DB is coupled to the switch SB and to the inductance LB while the cathode of the diode DB is coupled to the output capacitor CO. The boost converter 4 provides an output voltage VOUT to the high-side of the H-bridge 6. The H-bridge 6 comprises high-side switches HS1 and HS2 that are coupled to the supply voltage VOUT and to a first node A and to a second node B, respectively. Further, the H-bridge 6 comprises low-side switches LS1 and LS2 that are coupled to the first and second node A and B, respectively. The respective opposite side of the low-side switches LS1 and LS2 is coupled to ground. Between the first node A and the second node B there is an exemplary capacitive load, namely an LCD-display (LCD). The LCD is schematically represented by an Resistance R and a load capacity C.
Charging of the LCD is performed by exemplarily applying an output voltage VOUT of e.g., 12 V, via the first high-side switch HS1 and the second low-side switch LS2 to the load capacitance C. Opposite charge states of the load capacitance C may be realized by crosswise activation of the respective high-side and low-side switches. For discharging the load capacitance C, the H-bridge 6 is set to a state where both high-side switches HS1, HS2 are open and both low-side switches LS1, LS2 are closed.
This scheme is usually applied to ensure proper operation of an LCD display. However, during the discharging steps, the energy/charge that is stored in the load capacitance C is lost. This results in a low efficiency of this driving scheme. Especially for battery-driven applications, this is not desirable.
Within the context of this specification, a closed switch or switch that is in a closed state is a conductive switch while an opened switch or switch in an opened state is a non-conductive switch.